This invention relates to capacitance measurement probes, and more particularly, to a capacitance probe and operational circuitry therefor.
Capacitance probes are often used to measure the level of a material in a tank or other compartment. As the material rises in the compartment, it replaces the air between two electrodes or conductors. If the material has a higher dielectric constant than air, the total capacitance of the system is increased as the compartment is filled. This increase in capacitance provides an indication of the amount of material in the compartment.
In order for capacitance probes to operate in this setting, a pair of conductors must be spaced such that the material to be measured may fill the space therebetween. If the probe is to be inserted into conductive materials, then it must also incorporate some method of electrically insulating the conductors from one another.
One capacitive apparatus, shown in U.S. Pat. No. 3,774,238 to Hardway, uses two long tubes or rods 26, 27 insulated from each other in a spaced apart relationship by plastic insulators 28.
Another type of capacitance probe, shown in U.S. Pat. No. 5,397,995 to Anderson, includes an outer conductor and a spaced inner conductor. The space between conductors insulates the conductors from one another and allows the material to be measured to fill the space.
A drawback of known capacitance probes is their susceptibility to inaccuracies due to capacitances, commonly known as xe2x80x98parasitic capacitancesxe2x80x99, that exist between various components of the capacitance measurement circuit, including the probe itself. Moreover, these parasitic capacitances are often variable, being influenced by ambient temperature and/or humidity. Thus, a need exists for an improved capacitance measurement probe and circuitry therefor.
According to an embodiment of this invention, a capacitance measurement circuit includes a selection/shield drive circuit having a power port couplable to a constant-current source, a detector port couplable to a threshold detector, a probe port couplable to a probe capacitor, and a reference port couplable to a reference capacitor. The circuit also includes a plurality of switches being actuatable to alternately couple said power port to said probe port and to said reference port. The selection/shield drive circuit is configured to alternately generate a signal of linear ramp waveform having a slope that is proportional to the magnitude of capacitance of a probe capacitor and reference capacitor coupled to the probe port and to the reference port.
In a variation, the present invention may include a capacitance probe including this capacitance measurement circuit.
In a further aspect of this invention, a capacitance measurement circuit includes a constant-current source, a threshold detector, a selection/shield drive circuit coupled to the constant-current source and to the threshold detector. The selection/shield drive circuit includes a probe port couplable to a probe capacitor, a reference port couplable to a reference capacitor, and a plurality of switches configured to alternately couple the constant-current source to the probe port and to the reference port. The selection/shield drive circuit is configured to generate a signal of linear ramp waveform having a slope that is proportional to the magnitude of capacitance of the probe capacitor and reference capacitor alternatively coupled thereto. The threshold detector is configured to indicate when the linear ramp waveform reaches a threshold voltage.
In yet another aspect of the invention, a method of fabricating a capacitance measurement circuit includes providing a selection/shield drive circuit having a power port couplable to a constant-current source, a detector port couplable to a threshold detector, a probe port couplable to a probe capacitor, and a reference port couplable to a reference capacitor. The drive circuit is also provided with a plurality of switches being actuatable to alternately couple said power port to said probe port and to said reference port. The method also includes configuring the selection/shield drive circuit to alternately generate a signal of linear ramp waveform having a slope that is proportional to the magnitude of capacitance of a probe capacitor and reference capacitor coupled to the probe port and to the reference port.
In another aspect, the present invention includes a method of determining the capacitance of a capacitance probe. The method includes providing a selection/shield drive circuit, coupling a constant-current source to the selection/shield drive circuit, coupling a probe capacitor to the selection/drive circuit, coupling a reference capacitor to the selection/drive circuit, and providing the selection/drive circuit with a plurality of switches being actuatable to alternately couple the constant-current source to the probe capacitor and to the reference capacitor. The method also includes configuring the selection/shield drive circuit to alternately generate a signal of linear ramp waveform having a slope that is proportional to the magnitude of capacitance of a probe capacitor and reference capacitor coupled to said probe port and to said reference port, and actuating the switches to alternately couple the constant-current source to the probe capacitor and to the reference capacitor. The selection/shield drive circuit is used to alternately generate a signal of linear ramp waveform having a slope that is proportional to the magnitude of capacitance of a probe capacitor and reference capacitor. A voltage differential of substantially zero volts is maintained between the reference capacitor and the probe capacitor during said use of the selection/shield drive circuit.